Sweat diffusion through snow: Canines and trapped humans

Ezine

Published: May 15, 2016

Author: Steve Down

Channels: Base Peak

Sniffing out victims under snow

Image: courtesy 1st Special Response Group

Search and rescue dogs are an unmatchable resource when teams are out looking for victims buried under snow. As soon as people are covered up, they are virtually invisible to humans who have to use poles to locate them. The dog, however, can regularly sniff them out at depths of 2-4 m, although extreme cases of canine detection down to 10 and 12 m have been reported.

Rapid location and recovery are essential because the chances of survival fall rapidly from 90% if people are found within 15 minutes, to 30% after 35 minutes and 3% after 2 hours. So, the role of the rescue dog is vital, especially considering that it can search far quicker than humans.

The dogs work by sniffing out traces of the body odour of the victims which diffuses through the snow to the surface. Human odour has been studied effectively and many of its components have been identified. Some of these are not unique to human scent but it is believed that a combination gives rise to the distinctive odour that dogs can sense.

How the odour components move through the packed snow to the surface is not clear, although they will be affected by factors such as hydrophilicity. In an effort to gain more understanding, scientists in Europe have devised a method for measuring the concentrations of two major human odorants in snow.

Snow core samples

Federico Dallo and colleagues from the University of Venice, Italy, and the University of Nice-Sophia Antipolis, France, concentrated on two particular volatile compounds which are found specifically in human odour. The first was 3-hydroxy-3-methylhexanoic acid (HMHA) which is secreted equally in the armpits of men and women and has a characteristic odour. The second compound, 3-methyl-3-sulfanylhexan-1-ol (3MSH), is gender specific, females excreting more than males.

The method relied on the removal of the two compounds from melted snow by direct immersion SPME which provided extraction efficiencies of 10-12 and 2-3% for HMHA and 3-MSH, respectively. Although relatively low, these were adequate for measuring concentrations down to 0.04 and 0.20 ng/mL for HMHA and 3MSH, respectively.

This mode of extraction, in which the fibre was submerged in the sample, was more precise than headspace SPME, even though the latter resulted in better extraction efficiency (7%) for 3MSH. It was also affected very little by the snow matrix itself, although, in practice, snow from different locations might have different matrix effects.

The analytes in the extracted samples were measured by GC/MS with selected ion monitoring which had detection limits of 0.01 and 0.06 ng/mL, respectively, for HMHA and 3MSH.

Diffusion behaviour differs for marker compounds

The GC/MS method was used to test diffusion of the odour compounds through a core of ice that was collected at Passo Rolle in Italy. A cotton pad soaked with HMHA and 3MSH was placed at the bottom of the core alongside a small heater set at 37°C to simulate the warmth of a human body.

The compounds diffused through the core which was sampled at 10-cm lengths from the bottom, melting the small sections that were cut out for direct immersion SPME and GC/MS. The measurements were used to construct vertical diffusion profiles for each compound which showed that they behaved differently as they moved through the snow. The researchers thought that the different hydrophobic behaviours of HMHA and 3MSH might be responsible but admitted that it was speculation at this stage.

The key finding is that that the method is capable of measuring the sweat biomarkers through snow and "opens up the possibility of studying and modelling the diffusion mechanism of these compounds through the snowpack towards the atmosphere," the team concluded.

This could lead to a better understanding of how search and rescue animals find their victims buried under snow, giving hope to those trapped after an avalanche, as well as walkers and skiers who have fallen and become covered with snow.